Java HashMap returning null when object and value are present [duplicate]

Question

I am trying to get a Point2D.Double out of a HashMap for a java game.

 public void Undo(){
    if(Moves.size() >=1)
    {
        GW.ClearBoard();
        Ships.clear();
        Move m =Moves.remove(0);
        HashMap<Ship,Point2D.Double> shipMoves = m.getShips();
        for(Ship s: shipMoves.keySet()){
            if(shipMoves.get(s)!= null){
                Point2D.Double pos = shipMoves.get(s);
                String type = s.getType();
                GW.setShipLocation(pos, type);
                s.setPosition(pos.x, pos.y);
                Ships.add(s);
            }
        }
        System.out.println("Undo");
    }
}

As you can see from the screen captures of me debugging in eclipse, the hash map contains an entry for key=MasterShip(id=87) with a value containing Point2D.Double and the input S is a ship with id=87 and yet the program still returns null and I have no idea why or what I can do about this. I have checked the values in the Move class through the debugger as well.

Correction, there would be images but apparently I cannot post images yet.

edit

You can also iterate over the entrySet() of the map. – Nick Hristov

Thank you, the entryset() method of solving this issue worked perfectly, dunno how to up-vote or set a comment as an answer though.

To start, use Java naming conventions (types start with a capital, methods and variables don't). It's difficult to tell at a glance whether some of your code is working with statics. — chrylis -cautiouslyoptimistic-
– chrylis -cautiouslyoptimistic-, Commented Nov 30, 2014 at 4:47
Does your Ship class override .equals()? You may have a different object reference, and therefore wouldn't be getting the object from the HashMap. — Foxsly
– Foxsly, Commented Nov 30, 2014 at 4:48

Vallabh Patade · Accepted Answer · 2014-11-30 05:11:00Z

1

Iterate over the EntrySet of the Map

   for(Map.Entry<Ship,Point2D.Double> e : shipMoves.entrySet(){
        if(shipMoves.get(s)!= null){
            Point2D.Double pos = e.getValue();
            String type = s.getType();
            GW.setShipLocation(pos, type);
            s.setPosition(pos.x, pos.y);
            Ships.add(s);
        }
    }

answered Nov 30, 2014 at 5:11

Vallabh Patade

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Community · Accepted Answer · 2017-05-23 11:57:56Z

-1

If you use a HashMap with an object as a key, you need to implement two methods:

boolean equals(Object other)
int hashCode()

Check out this question: Creating a hashCode() Method - Java

edited May 23, 2017 at 11:57

CommunityBot

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answered Nov 30, 2014 at 4:49

Anubian Noob

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2 Comments

Bohemian Over a year ago

It should still work without defining these methods - the same exact object will be equal() to itself and have a consistent hash code. What won't work is matching an "equal" key object (one with same field values).

Duncan Tyrell Over a year ago

I am getting the keyset out of the hashmap so it kinda has to be exactly equal to the key does it not? I really don't understand what I have to do to get this to work.

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